The purpose of this project is: (1) to investigate age-related changes in thermoregulation; and, (2) to examine the physiological mechanisms underlying these changes. We have demonstrated that aged mice have diminished cold tolerance and are not able to adapt to repeated cold exposure. The cause of these age- related aberrations in thermoregulation appears to be, in part, a reduction in metabolic heat production due to change in brown adipose issue (BAT) and, in part, a reduction in heat conservation. Efferent sympathetic nervous responses to BAT are enhanced in both cold- acclimated and aged animals, which suggests that the sympathetic nervous system plays a major role in cold acclimation, but is not responsible for the aged-related decline in thermoregulation. Our results show that sympathetic nervous activity increases with age to adapt to age-related declines in effector organ function.